2. Field of the Invention
This invention relates to a method for producing high density sintered silicon nitride (Si.sub.3 N.sub.4) having a density equivalent to or higher than that of a conventional hot pressed body, which method facilitates the production of sintered bodies of complex configurations. More particularly, it relates to a method of further desifying and strengthening a presintered silicon nitride body, which has in advance been formed and presintered into a desired shape by the pressureless sintering method or the like, in accordance with the hot isostatic pressing method (hereinafter referred to as "HIP method").
2. Description of the Prior Art
With a view toward improving the thermal efficiency, saving fuel, reducing pollution and lightening the weight of varied power generating equipment, active research and development work has been carried out in recent years in the field of equipments which are operated at high temperatures led by high temperature gas turbines and followed by diesel engines and MHD generators. Development of such equipments is absolutely dependent on the development of high-temperature structural materials. Thus, the development of such materials has been anxiously waited for. Under such high temperatures, conventional heat resistant metallic materials do not always exhibit sufficient mechanical strength. Also, from the view-point of saving exiguous natural resourses required for the production of such heat resistant metallic materials, intensive researches are now under way to develop high-temperature structural ceramics using as raw materials Si, Al, C, N and the like elements which are relatively abundant on the earth.
The development of such high-temperature structural materials has been recognized to be also important for the purpose of their applications in super hard tools and as corrosion resistant materials, and is attracting great interests thereon.
Among such high-temperature structural ceramics, silicon nitride (Si.sub.3 N.sub.4) is attracting researchers interests as one of most promising materials for providing sufficient strength, chemical stability and strong resistance to thermal shocks at high temperatures.
Si.sub.3 N.sub.4 pertains excellent physical properties as described above. These properties stem from the fact that silicon atoms (Si) and nitrogen atoms (N) are united by firm covalent bonds in silicon nitride (Si.sub.3 N.sub.4). However, this nature of Si.sub.3 N.sub.4 means, on the other hand, that it is hard to be sintered and extremely difficult to be formed into a product of complex configurations. As a matter of fact, most of recent researches in this field are directed how to produce formed Si.sub.3 N.sub.4 bodies having high strength. However, in view of the present state of the art, it does not appear that a fully satisfactory technique has been developed for the production of such Si.sub.3 N.sub.4 bodies.
More specifically, where high density and high strength are pursued, the shape of a formed body has to be unavoidably limited to simple one. On the other hand, to obtain a formed body of complex configurations, its strength has to be sacrificed to a considerable extent.
Among well-known conventional production methods of formed Si.sub.3 N.sub.4 bodies, there are following four methods: (i) the chemical vapor deposition method (hereinafter referred to as the CVD method); (ii) a method in which Si.sub.3 N.sub.4 powder is mixed with a sintering aid and then sintered in an N.sub.2 atmosphere of atmospheric pressure or 10 atom or so; (iii) the hot pressing method; and (iv) reaction bonding method. Of the above methods, the hot pressing method (iii) can provide formed bodies of a relatively high density and strength but is still accompanied by problems that formed bodies of a complex shape are difficult to obtain and it is costly to practise.
On the other hand, the reaction bonding method has a merit that complex configurations can easily be formed by a suitable conventional method owing to the use of Si powder as a raw material. However, it is accompanied by drawbacks that resultant sintered bodies do not have a high density and high strength sintered Si.sub.3 N.sub.4 cannot be obtained. The densities of sintered Si.sub.3 N.sub.4 bodies currently produced in accordance with the reaction bonding method are merely somewhat greater than 80%. This insufficient density impedes to improve the strength of sintered bodies. Moreover, an extremely long time period is required for the nitridation. For example, the reaction treatment requires at least two days in shorter cases and, in some longer cases, takes as long as 10 days or more. This is certainly a great problem in adopting the reaction bonding method.
The pressureless sintering method is a method to sinter a silicon nitride green compact which has in advance been formed into a desired shape. Thus, this method is rather easy to produce sintered products of complex configurations. However, the densities of such products are limited and are around 96% even for higher ones.
As an improved method over the above-described, pressureless sintering method, there has been proposed to sinter a green compact, which has been preformed into a desired shape, in an N.sub.2 gas atmosphere of several atm to several ten atm as described in Japanese Patent Laid-open No. 47015/1972 laid-open Apr. 14, 1972 and naming as an inventor Mamoru Mitomo as well as Japanese Patent Laid-open No. 102320/1928laid-open Sep. 6, 1978 and naming as an applicant General Bleatrio Company. Such an improved method can provide sintered bodies having a density as high as 98% at maximum. However, it still fails to meet both of the requirements, namely, requirements for the formation of complicated shapes and the high densification. Especially, as a serious obstacle to the high densification, there is mentioned the thermal decomposition problem of Si.sub.3 N.sub.4 upon sintering. Namely, upon sintering Si.sub.3 N.sub.4, MgO, SiO.sub.2, Al.sub.2 O.sub.3 and/or the like are incorporated as a sintering aid. These compounds are however believed to volatilize during the sintering step by their reaction with Si.sub.3 N.sub.4 as described below. ##STR1##
On the other hand, Si.sub.3 N.sub.4 per se is known to undergo a thermal decomposition as described below. ##STR2##
Due to these thermal decomposition reactions, a weight loss normally takes place during a sintering step and, in some instance, such a weight loss exceeds the rate of density increase due to the shrinkage of a sintered body upon sintering. It has been reported that such a weight reduction may, in some instances, reach as high as 50%. The above-described sintering method in an N.sub.2 gas atmosphere was proposed as a measure for inhibiting the thermal decomposition but is still believed to cause a weight loss of several percents.
According to a research carried out by the present inventors, it has been recognized that the weight loss caused by the thermal decomposition has a close relationship with the density of a green compact before sintering as will be described later. The present inventors have also found that the weight loss due to thermal decomposition tends to become considerable where the initial density (i.e., the density of a green compact before sintering) is low but it becomes smaller as the initial density increases. From this finding, the decomposition reaction is considered to proceed, in the conventional sintering method in an N.sub.2 gas atmosphere, to the interior of a sintered body since the density of a green compact before sintering is merely 60% or so and its pores are completely open, and reaction products of the thermal decomposition are thus allowed to diffuse from the interior of the sintered body to the exterior of the same.
It is certainly an effective measure to increase the nitrogen gas partial pressure to suppress the aforementioned thermal decomposition in the sintering method in an N.sub.2 gas atmosphere. However, an N.sub.2 gas partial pressure of several ten atm or so, which has been employed in the conventional method, is by no means sufficient to effectively prohibit the thermal decomposition from the thermodynamical viewpoint.
As a method for highly densifying Si.sub.3 N.sub.4, it has been known to employ an HIP treatment using Ar gas. To follow this method, Si.sub.3 N.sub.4 powder is generally sealed hermetically in a capsule made of a gas-impermeable material such as glass and then subjected to an HIP treatment. This method is however impractical to produce a sintered body of a complex shape as it is extremely difficult to shape a capsule corresponding with the configurations of the intended product. It is also impractical as it still involves many problems to be solved, such as filling uniformly Si.sub.3 N.sub.4 powder in capsules, measures to avoid the reaction between Si.sub.3 N.sub.4 and capsules, and measures to decapsulate resultant Si.sub.3 N.sub.4 bodies from the capsules. In addition, it has also been proposed to close the pores of a green compact to a certain extent prior to applying an HIP treatment by subjecting the green compact to a presintering treatment. It is however difficult, generally speaking, to achieve a high density by this method as a considerable weight loss takes place due to the thermal decomposition of Si.sub.3 N.sub.4.
On the other hand, a lot of investiment has been poured to find out a sintering aid suitable for producing high density sintered Si.sub.3 N.sub.4. However, under the present circumstances, it is still far away from the goal. It is considered to be promising to employ Y.sub.2 O.sub.3 or a mixture of Y.sub.2 O.sub.3 and Al.sub.2 O.sub.3 as a sintering aid for Si.sub.3 N.sub.4. As a matter of fact, it has been known that a sintered body having excellent mechanical properties can be obtained by mixing a suitable amount of such a sintering aid and Si.sub.3 N.sub.4 powder and by sintering the resultant mixture in accordance with the hot pressing method. However, such Y.sub.2 O.sub.3 system sintering aids are accompanied by a drawback that the pressureless sintering method, which is rather easy to produce bodies of complex configurations, cannot be applied to obtain sintered bodies of high density and strength. Thus, such Y.sub.2 O.sub.3 system sintering aids are employed exclusively for the production of bodies of simple configurations in accordance with the hot pressing method.
As a result of an extensive research conducted by the present inventors, it has been unexpectedly found that high density sintered silicon nitride (Si.sub.3 N.sub.4) of a relative density of 98% or higher may be obtained by compacting silicon nitride powder into a green compact having a desired shape, presintering the green compact to a relative density of at least 92%, and then subjecting said presintered body to a hot isostatic pressing in an inert gas atmosphere of a temperature in the range of 1500.degree.-2100.degree. C. and of a nitrogen gas partial pressure of at least 500 atm until the former relative density is reached. In this invention the gas used in the hot isostatic pressing is only the nitrogen or a gas mixture including the nitrogen. This method may be similar to the aforementioned prior art methods in an incorporation of the HIP method. However, the above finding of the present inventors is fundamentally different from the conventional HIP method as, in the former case, the relative density of a presintered body is raised to 92% or higher and it is then subjected to an HIP treatment in an inert atmosphere of the above specific temperature and N.sub.2 gas partial pressure.
The present inventors have also found that an incorporation of a Y.sub.2 O.sub.3 --Al.sub.2 O.sub.3 --MgO system mixture as a sintering aid in Si.sub.3 N.sub.4 powder can provide under normal pressure a presintered body of a higher density compared with the addition of a conventional Y.sub.2 O.sub.3 or Y.sub.2 O.sub.3 --Al.sub.2 O.sub.3 system sintering aid.
As a result of an experiment carried out by the present inventors to determine Whether there is any relationship between the relative density of sintered Si.sub.3 N.sub.4 and its strength, it has been found that they are not always corelated to each other. The inventors expanded the study and carried out a further experiment while paying attention to .beta.--Si.sub.3 N.sub.4 in presintered bodies. As a result of the further experiment, it has been found that the strength of a sintered Si.sub.3 N.sub.4 body corelates to the content of .beta.--Si.sub.3 N.sub.4 in the presintered Si.sub.3 N.sub.4.
Furthermore, it has been realized that the present sintering method, which makes use of an HIP treatment, involves the following problem in order to more effectively apply the same to the industry. Namely, a presintered body is discharged from a presintering furnace upon completion of the presintering step and cooled prior to charging the same into an HIP furnace. The thus-presintered body is then charged at a cooled temperature into the HIP furnace, although the HIP treatment requires 1000.degree. C. or higher, particularly, a high temperature of at least 1500.degree. C. for Si.sub.3 N.sub.4.
Such a cooling of a presintered body maybe unavoidable where the sintering method is performed batch by batch. This however leads undoubtedly to a considerable loss of heat energy in view of the fact that the presintered body has been heated to a considerable extent during its presintering step. In addition, when a presintered body which has been presintered at high temperatures is rapidly cooled at the surface thereof or after the rapid cooling when rapidly heated extremely fine cracks and fissures are susceptible of occurring, whereby unavoidably deteriorating its strength and causing other problems with respect to its quality.
The present inventors have also found a measure capable of effectively overcoming the above problems of the thermal energy loss and strength deterioration caused by the rapid cooling of presintered and sintered bodies. Namely, it has been uncovered that the thermal energy loss may be reduced and a high density sintered Si.sub.3 N.sub.4 body can be obtained by charging a presintered body, while maintaining its temperature above 500.degree. C., into an HIP furnace which has been in advance heated at 500.degree. C. or higher to conduct the HIP treatment, discharging the thus-sintered Si.sub.3 N.sub.4 body from the HIP furnace at a temperature of at least 500.degree. C., and then subjecting the same to a heat treatment at 500.degree. C. or higher in a heat treatment furnace.